In recent years, the industrial utilization of porous glasses as adsorbing agents, microcarrier supports, separation films, optical materials, and the like has been highly anticipated. In particular, porous glasses have a wide utilization range as optical members because of a characteristic of low refractive index.
As for a method for manufacturing a porous glass relatively easily, a method taking advantage of a phase separation phenomenon has been mentioned. A typical example of a base material for the porous glass exhibiting the phase separation phenomenon is borosilicate glass made from silicon oxide, boron oxide, an alkali metal oxide, and the like. In production, the phase separation phenomenon is induced by a heat treatment in which a molded borosilicate glass is held at a constant temperature (hereafter referred to as a phase separation heat treatment), and a non-silicon oxide rich phase, which is a soluble component, is eluted through etching with an acid solution. The skeleton constituting the thus produced porous glass is primarily silicon oxide. The skeleton diameter, the hole diameter, and the porosity of the porous glass are affected by the composition before the phase separation heat treatment is performed and the temperature and time of the phase separation heat treatment significantly. The skeleton diameter, the hole diameter, and the porosity have influences on the reflectance and the refractive index of the light.
PTL 1 discloses a method for forming a porous glass film on a base member. Specifically, a phase-separable base material glass film is formed on the base member by applying and fusing a film including phase-separable borosilicate glass particles on the base member, and the porous glass film is formed on the base member by a phase separation heat treatment and an etching treatment.
In order to utilize a porous glass as an optical material, it is required that no bubbles causing scattering of light are included. In a known method for removing bubbles, glass is heated at a high temperature of 1,350 degrees (celsius) to 1,500 degrees (celsius), as described in NPL 1.